Natural and Unnatural Disasters

As New Orleans and the other stricken communities of the Gulf Coast
begin rebuilding, they will doubtless give much thought to
protecting themselves from the next big storm. Levees and flood
walls will be strengthened, perhaps the plan to install storm gates
at the entrances to Lake Pontchartrain will be revived, and work to
restore coastal wetlands may finally win funding. Protective
measures for the beachfront communities of the Mississippi coast are
harder to engineer, but perhaps this occasion will be taken as an
opportunity to move at least some structures farther from the surf zone.

Given the century-long history of "back door" flooding in
New Orleans, the Corps of Engineers and other flood-control agencies
may well be criticized for devoting too much energy to the
Mississippi River while neglecting the hurricane hazard. But in fact
the river remains the greatest force of nature in the region. Before
Katrina, the worst disaster in Louisiana—and one of the worst
in the nation's history—was the Mississippi flood of 1927, in
which nearly a million people were forced from their homes. In that
case the city of New Orleans was spared—but only by dynamiting
a levee downstream, wiping out much of Plaquemines Parish. On the
Lower Mississippi, floods of this magnitude can happen only when
several major tributaries (the Ohio, the Upper Mississippi, the
Missouri, the Arkansas) all rise at once. Such an unfortunate
coincidence is rare, but eventually it is bound to happen again.

Ironically, New Orleans also faces the contrary risk: being left
high and dry. Two hundred miles upstream, the Old River Control
Structure diverts flood waters into the Atchafalaya River, but the
gates and weirs there have another purpose as well: They are
intended to prevent the Atchafalaya from "capturing" the
entire flow of the Mississippi, leaving New Orleans and Baton Rouge
stranded on a stagnant backwater. The Atchafalaya takes a shorter
and steeper route to the Gulf, and so physics is on its side. In
1973 the Control Structure was nearly washed away; if it had failed,
the outlet of the Mississippi would now be at Morgan
City—except that Morgan City would also have been washed away.
Following that close call, the Corps of Engineers added another
complex of gates at Old River. It is a great mass of concrete and
steel, and yet it looks delicate enough when you reflect that whole
cities depend on this structure for their wellbeing and maybe their existence.

Editor's note: Brian Hayes, whose "Computing Science"
customarily appears in this space, became acquainted with flood
control on the Mississippi while writing his just-published
book Infrastructure: A Field Guide to the Industrial Landscape
(W. W. Norton). After Hurricane Katrina struck, we asked him
to contribute a short essay on the battered infrastructure of
New Orleans. "Computing Science" will return next issue.